High density circuit modules in computers and other electronic apparatus generate significant amounts of heat which, if not removed from the circuit, can cause the circuit components to rise to temperatures beyond their rated capacity. Also, even within its rated capacity, circuits commonly can operate at much higher speeds and provide much greater performance at cooler temperatures. For example, in bi-CMOS circuits, if the junction temperature of the silicon die is maintained at -5.degree. C. to 10.degree. C., circuit speed can be increased approximately 50%. Numerous apparatus are available for cooling circuit modules. The particular design used in any given situation is highly dependent upon many factors including the amount of heat generated by the circuitry, the ambient temperatures in which the circuitry is to be used, the space available for cooling, hardware, etc.
One known apparatus used in circuit cooling structures is a thermo-electric device. A thermo electric device typically comprises a set of alternately negatively and positively doped semiconductor regions electrically coupled in series and sandwiched between two electrically insulative and thermally conductive layers (Al.sub.2 O.sub.3 ceramic substrates). When current flows through the semiconductors, heat is conducted from one of the dielectric layers (the cold layer) to the other dielectric layer (the hot layer). In a typical application of a thermo electric device, the cold layer is physically coupled to the module to be cooled while the hot layer is coupled to a thermally conductive heat sink.
FIG. 1 illustrates an exemplary circuit cooling apparatus including a thermo-electric device. A circuit module is shown generally at 12. Circuit module 12 includes a ceramic substrate layer 80. Circuit layer 82 is bonded to substrate 80. FIG. 1 shows a second circuit layer 84 bonded to layer 82, however, the desired circuit design may require more or less than two circuit layer. Finally, silicon die 86 is coupled to the circuit layer 84. The die 86 may be coupled to the circuit layer 84 by epoxy or it may be reflowed with a tin-gold eutectic solder. Other methods are also known. The silicon die 86 can be wire bonded or TAP bonded to the circuit layer 84 for the required electrical connections.
The substrate layer 80 of circuit 12 is coupled to a copper cold plate 14 at interface 16. A layer of thermal grease is applied between the substrate 80 and the cold plate 14 at interface 16. The thermal grease provides a thermally uniform coupling between the substrate 80 and cold plate 14. For instance, the substrate 80 and the cold plate 14 may have their mating surfaces milled flat to approximately .+-.1/1000 of an inch. Due to irregularities in the mating surfaces of .+-.1/1000 of an inch or less, the two surfaces do not uniformly contact at all places thus forming air gaps in interface 16. The thermal grease fills the air gaps between the mating surfaces of substrate 80 and cold plate 14 providing uniform thermal coupling between the two components. The thermal grease interface 16 is maintained by fixing the circuit module 12 to the copper cold plate 14 by mechanical means such as screws or clamps (not shown).
A thermo-electric device 18 is coupled to the opposite face of cold plate 14, forming interface 20. Thermal grease also is employed in interface 20 to provide uniform thermal coupling between the mating surfaces of thermo-electric device 18 and cold plate 14. An exemplary thermo-electric device sometimes referred to as a heat pump is, Part No. 2CP110050-32(32) manufactured by Material Electronic Products Corp. of Trenton, N.J. This type of device conducts heat in a specified direction by means of the Peltier Effect, a known physical effect by which heat is evolved or absorbed at the junction of two dissimilar metals which carry a small current. Layer 22 of thermo-electric device 18 comprises a thermally conductive dielectric such as alumina, Al.sub.2 O.sub.3, which is a ceramic. Copper cold plate 14 is coupled to the alumina layer 22 of thermo electric device 18 by mechanical means. This second layer of thermal grease (interface 20) is applied between the cold plate 14 and the alumina layer 22 to eliminate air gaps and provide uniform thermal coupling. Typically, the circuit module 12, the cold plate 14 and the thermo-electric device 18 are coupled together by a single clamping means which holds all three parts together. Within thermo-electric device 18, the next layer is a layer of etched copper 24. The etched copper layer 24 is sometimes referred to as a bus-bar and is coupled to the alumina 22 at interface 26 by means of a lamination process. The etched copper layer 24 is attached at its face opposite alumina layer 22 to a layer of alternately positively and negatively doped semiconductor regions generally represented at 28 in FIG. 1. The alternating positively doped regions 30 and negatively doped regions 32 preferably comprise high grade bismuth telluride in the form of oriented polycrystalline ingots. The semiconductor regions are coupled to the etched copper layer 24 by solder joints 34 which preferably comprise indium tin solder. A second etched copper layer 36 is placed on the other side of semiconductor layer 28. The semiconductor regions are coupled thereto by means of additional indium tin solder joints 34. The etched copper layers (or bus-bars) 24 and 36 are etched to provide a conductive path in which the semiconductor regions are coupled in a series pattern such that current flows successively through a positive semiconductor region 30 in a downward direction from copper layer 36 to copper layer 24, and through a negatively doped semiconductor regions 32 in an upward direction from copper layer 24 to copper layer 36 and so on. Due to the Peltier Effect, the flow of current through the semiconductor regions causes heat to flow upwardly in FIG. 1, away from first alumina layer 22. The upper etched copper layer 36 is coupled to a second alumina layer 38 which receives the heat conducted away from first alumina layer 22. Once again, alumina layer 38 is bonded to the etched copper layer 36 at interlace 40 by a lamination process. Finally, alumina layer 38 is coupled at its face opposite etched copper layer 36 to a heat sink 42. Heat sink 42 typically comprises a heavy mass of thermally conductive metal such as aluminum or copper which is cast in a shape having a very high surface area to mass ratio. Another layer of thermal grease is applied between alumina layer 38 and the heat sink 42 to eliminate the effects of the physical imperfections of the two lapped, mating surfaces. Alumina layer 38 is coupled to the heat sink 42 by mechanical means such as screws 46. In fact, it is common to provide mechanical clamping means, such as screws, to hold together the entire circuit structure including circuit module 12, cold plate 14, thermo-electric device 18 and heat sink 42. A fan may be provided (not shown) to force cooling air through veins 48 of the heat sink 42. In some situations, liquids may be used.
In this prior art circuit cooling apparatus, the multiple interfaces between the various layers, and particularly the interfaces containing thermal grease such as interfaces 16, 20 and 44 create a significant amount of resistance to the flow of heat. It is desirable to reduce the number of layers between the actual circuitry, i.e., the silicon dies and circuit layers of circuit module 12 and the heat sink 42 to increase thermal efficiency.
Therefore, it is an object of the present invention to provide a heat sink apparatus having a reduced number of thermally-resistive layers, thereby reducing the power used by the thermo-electric device to maintain a specified circuit temperature.
It is a further object of the present invention to provide a heat sink apparatus which does not require thermal grease in any of the interfaces between layers.
It is another object of the present invention to provide an improved heat sink apparatus.
It is one more object of the present invention to provide a heat sink apparatus which requires no mechanical means for coupling the various layers together.
It is yet another object of the present invention to provide a heat sink apparatus having increased thermal efficiency.